The search for the biological alterations that occur in the brain that are substrates for learning and memory has proved to be one of the most elusive in science. Several important discoveries have led to the initial localization of neural circuits putatively involved in certain classically conditioned responses. Recently,convergent lesion/behav- ioral, electrophysiological and anatomical data have established the midline cerebellum as essential for the acquisition and retention of conditioned bradycardia in the rabbit. The proposed research is based on the hypothesis that the midline cerebellum, along with the central n. of the amygdala (ACE), are essential components of a more extensive neuro- anatomical system involved in the acquisition and retention of this autonomic conditioned response. We hypothesize it is through anatomical connections with brainstem cardioregulatory nuclei that the midline cerebellum makes these essential functional contributions to conditioned bradycardia. In keeping with this hypothesis, the objective of the present research is to further our understanding of the functional contributions of the cerebellum to autonomic conditioning by studying the anatomical and neurophysiological interactions between the midline cerebellum and the pontine parabrachial n. (PBN), a brainstem cardioregulatory n. with (i) direct anatomical connections with the midline cerebellum an( (ii) direct anatomical connections with a forebrain n. importantly involved in conditioned bradycardia, the ACE. Toward this objective, the present research has been designed to (a) determine precisely the anatomical relationships between the midline cerebellum and the PBN, (b) characterize the neurophysiological responses and interactions between neurons in these regions, and the alterations which may occur as a result of aversive Pavlovian conditioning and, (c) the effects of neurotoxic lesions of components of this cerebellar-parabrachial circuit on the acquisition and retention of conditioned bradycardia in the rabbit. These data will considerably extend our knowledge of the manner in which cerebellar neurons interact with other brain regions during conditioned bradycardia, and enhance our understanding of brain function by identifying, more exactly, a specific brain system involved in learning and memory. Such an identification is a necessary prerequisite to a more complete understanding of the ultrastructural and molecular substrates of learning and memory; processes basic to the survival of the organism. These data will be exciting given very recent evidence of cerebellar involvement in complex human learning and emotional processes.